Solid-state light emitting diodes (LEDs) have a prospected bright future for replacing conventional light sources in various lighting applications. More specifically, high-brightness inorganic LEDs are entering markets like automotive lighting, camera flashes, display projection and display backlighting.
The benefits of using these LEDs instead of conventional light sources are given by their small-volume, high luminance and high color saturation.
However, as conventional light sources, LEDs emit non-polarized light, i.e. light that does not posses a significant preference for a specific polarization state. Therefore, in applications requiring polarized light this light must be polarized with other means.
Application areas where polarized light is used are in LCD-backlighting and LCD-projection as well as in options for LC-beam steering devices in which the light beam emitted by LED point sources is manipulated with LC cells.
Also, polarized light yields advantages in both indoor and outdoor illumination as linearly polarized light influences reflections on surfaces which enable the suppression of glare and the subsequent influencing of observation of the illuminated surrounding in visual acuity, observed contrast and color saturation. Because of this influence, polarizing fluorescent luminaries exist as commercial products with a claimed benefit in visual perception.
It is possible to achieve a polarized light emission from the LED by using exotic crystal orientation directions in the fabrication of LEDs. However, such LEDs are difficult to manufacture and it is hard to obtain an efficient light output. Also, the reported polarized contrasts are small.
The common method for polarizing light involves the step of absorbing the unwanted polarization state. This method has an efficiency of about 45%.
US 2006/0066192 A1 discloses an illumination system that incorporates a LED and a reflecting polarizer. The reflecting polarizer transmits a first portion of light emitted from the LED, and reflects a second portion of the light emitted from the LED. The reflected light is incident on the LED, and reflected by the LED. Further, the second portion of light is scattered so that the polarization state of the reflected light is mixed. Thus, parts of the scattered light can be transmitted by the reflecting polarizer so that the total efficiency of the illumination system is increased.
Even if the system disclosed in US 2006/0066192 A1 shows an increased efficiency of polarization, it is not sufficient for many applications.